1. Field of the Invention
This disclosure relates to a signal processing device.
2. Description of the Related Art
A semiconductor integrated circuit operable with a low power voltage has been developed in order to lower power consumption. Meanwhile, the semiconductor integrated circuit includes a plurality of circuit blocks performing diverse functions and thus plural kinds of power supply voltage are needed for these circuit blocks.
In the case where transistors using different semiconductors are combined in one circuit block or in the case where in a circuit composed of transistors using the same semiconductors, their driving methods or their required characteristics are different, plural kinds of power supply voltage are sometimes needed.
Further, a power supply voltage different from that in an internal circuit is sometimes needed because a signal level of an input/output terminal connected to the outside of the semiconductor integrated circuit depends on the characteristics of an element to be connected.
Thus, plural kinds of power supply voltage are supplied to the semiconductor integrated circuit, and a circuit unit for converting a signal level, which is called level shifter, is provided in a portion over the chip where regions using different power supply voltages are in contact with each other or an output portion (e.g., Patent Document 1 and Non-Patent Document 1).
The level shifter disclosed in Non-Patent Document 1 is shown in FIG. 20. A level shifter 400 is composed of a total of 14 transistors (transistors 401 to 414). The level shifter 400 is supplied with a signal IN and an inverted single INB of the signal IN, which are low-voltage signals.
The amplitude of the signal IN and the inverted signal INB is VDD1-GND. OUT is a high-voltage signal which is output from the level shifter and whose amplitude is VDD2-GND. Note that VDD1 is a high potential used in a low-voltage circuit (e.g., internal circuit) and is higher than GND and lower than VDD2 (i.e., GND<VDD1<VDD2).
The level shifter 400 is a differential amplifier circuit composed of the transistors 408 to 411 and converts the amplitude of the signal IN from VDD1-GND to VDD2-GND. Because a low potential is GND in the differential amplifier circuit of the level shifter 400, if the amplitude voltage of the signal IN is lower than the absolute value of the threshold voltage of the transistors 408 to 411 (hereinafter, “threshold voltage” means “the absolute value of the threshold voltage” unless otherwise specified), there are problems of unstable amplification, the increase of noise effect, inability to convert signal at high speed, generation of leakage current, and the increase in power consumption.